Document GB-A-2,283,036 discloses water-based drilling fluids containing a polyalkylene glycol and an ester that can be a partial ester of a polyol with a fatty acid; it is claimed that these fluids are intended to be used in formations of clayey shales and clays, and that they combine a high lubricity and a high shale inhibiting power. The examples disclose the following partial esters:                glyceryl monooleate;        tetraethylene glycol monooleate; and        glyceryl monoricinoleate.        
However, example 7 clearly shows that addition of less than 3% by weight of mixture (2.25% by weight polyalkylene glycol and 0.75% by weight glycerol monooleate) does not provide high lubricity properties.
Document U.S. Pat. No. 5,318,956 (corresponding to document EP-A-0,398,113) discloses the use of esters of monofunctional or polyfunctional alcohol with saturated or unsaturated carboxylic acids up to C36 as the main constituent of the oil phase of drilling fluids of “oil-in-water” (O/W) emulsion type.
Document U.S. Pat. No. 3,882,029 discloses a completion and workover fluid comprising finely divided solid particles consisting of a homogeneous solid solution of:                wax;        a partial ester of glycerol or sorbitan with a high saturated fatty acid (C12–C20), which is oil-soluble and has surfactant properties;        a monoester of polyethylene glycol with a high saturated fatty acid (C12–C20), dispersible in water and having surfactant properties;        an ethylene-vinyl acetate copolymer; and        a fatty alcohol.        
Document U.S. Pat. No. 3,979,304 discloses a method for reducing the permeability of permeable underground formations, which consists in adding to the drilling fluid particles of a homogeneous solid solution comprising:                a wax;        a partial ester of a polyhydroxyl compound with a high fatty acid (C12–C20), which is oil-soluble and has surfactant properties;        a water-soluble surfactant, that can be a monoester of polyethylene glycol with a high fatty acid (C12–C20).        
Document WO-A-96/40,835 discloses “oil-in-water” (O/W) emulsions comprising a brine, a C7–C23 fatty acid triglyceride, an alcohol, an emulsifier and a sulfur-containing compound.